The present application relates to the field of imaging, such as particle detection, radiographic imaging, photography, etc. It finds particular application with medical, security, and/or other applications where measuring discrete events to obtain an image of an object may be useful, but it is not intended to be limited to merely the same.
Imaging systems, such as computed tomography (CT) systems, line scanners, etc., provide information, or images, of an object under examination (e.g., interior aspects an object under examination). For example, in radiographic imaging systems, the object is exposed to radiation, and one or more images are formed based upon the radiation absorbed by the object, or rather an amount of radiation that is able to pass through the object. Typically, highly dense objects absorb (e.g., attenuate) more radiation than less dense objects, and thus an object having a higher density, such as a bone or gun, for example, will be apparent when surrounded by less dense objects, such as fatty tissue or clothing, for example. A detector array, generally positioned opposite a radiation source from which radiation is emitted relative the object under examination, is configured to detect radiation that traverses the object under examination and convert such radiation into signals and/or data that may be processed to produce the image(s). Such an image(s) may be viewed by security personnel to detect threat items (e.g., weapons, etc.) and/or viewed by medical personnel to detect medical conditions (e.g., cancerous tissue), for example.
In some scanners, such as three-dimensional radiographic imaging scanners (e.g., CT scanners), for example, the detector array and radiation source are mounted on opposing sides of a rotating gantry that forms a ring, or toroid, around the object under examination. In such a scanner, the rotating gantry (including the radiation source and/or detector array) is rotated in a circle situated within an x, y plane about a z-axis substantially perpendicular to the x, y plane (e.g., an “isocenter”) during an examination. The object is generally supported by a support article (e.g., a bed, conveyor belt, etc.) that runs in the z direction substantially parallel to the mechanical center of rotation (e.g., the isocenter). As the rotating gantry is rotated, radiation is substantially continuously emitted from a focal spot of the radiation source toward the object under examination.
Radiation photons that impinge the detector array can be converted into a current or voltage, and a corresponding analog signal, that is representative of, or directly proportional to, the number of photons detected can be produced. For example, a higher voltage signal may correspond to more detected photons and a lower voltage signal may correspond to fewer detected photons.
Analog to digital converters (A/D) are often used in imaging systems to convert an analog signal, such as a voltage signal proportional to a number of detected photons, to a digital signal, where a digital signal may be more easily used for image reconstruction, for example. One type of ADC is the ramp ADC. An unknown input (e.g., indicative of a number of detected photons) is compared against a known input (e.g., a ramp signal), and a time it takes for the two signals to meet (e.g., have equal voltage levels) can be used as the digital output (e.g., number of timed sampling units). One of the more common types of ramp signals is a linear ramp signal, which is characterized by a voltage that increases at a constant rate. While this type of ramp has proven useful, it has several disadvantages. For example, identifying the voltage of an unknown signal from a linear ramp signal is time consuming because the sampling units (e.g., codes) are evenly distributed even though a non-uniform distribution of fewer units may provide the same or similar accuracy. For example, where a relatively large number of photons are detected (e.g., hundreds of thousands), fewer sampling units are necessary because the effect of missing a few photons (e.g., due to a lack of samplings) may be relatively insignificant given the large noise inherent to the signal and the large number of photons detected (e.g., as indicated by higher voltage signals). On the other hand, where a relatively small number of photons are detected (e.g., less than one hundred), more sampling units are necessary because the effect of missing a few photons (e.g., due to a lack of samplings) may be relatively significant given the relatively small photon noise and the small number of photons detected (e.g., as indicated by lower voltage signals). Thus, because the number of sampling units used for higher voltage signals (e.g., indicative of more photons) is generally the same as the number of sampling units used for lower voltage signals (e.g., indicative of fewer photons), more time is taken than is needed in conventional systems because fewer sampling units are needed for the higher voltage signals. Moreover, because of the high number of sampling units, the amount of data that is produced is large and may need to be compressed before transmission from a rotating gantry portion of a scanner to a stationary portion, for example, requiring additional resources and/or overhead.
More recently, non-linear ramp signals, such as logarithmic or square ramp signals, have been utilized because these ramp signals typically overcome at least some of the disadvantages of linear ramp signals. For example, non-linear ramp signals have been used because such signals generally require fewer sampling units, and thus take less time. Stated differently, at levels indicative of more photons (e.g., higher voltage signals) the number of sampling units used is less than the number of sampling units used at levels indicative of fewer photons (e.g., lower voltage signals), thus reducing overall time. Moreover, because the number of sampling units is fewer, the amount of data generated from a scan is less than the amount of data generated if a linear ramp where used. Thus, little to no data compression may be required to transfer the data from a rotating gantry portion of the scanner to a stationary portion of the scanner, for example.
While non-linear ramp signals overcome many of the disadvantages of linear ramp signals, non-linear ramp signals also have some disadvantages. For example, these ramps require high bandwidth circuits to generate the ramps quickly and accurately. Such circuits are costly to manufacturer and may introduce electronic and/or other types of noise into the system.